1. Field of the Invention
The present invention relates generally to integrated circuit packaging and, more particularly, high density micro lead frame packages.
2. Related Art
Recently developed packages for integrated circuit (IC) die or chips include the quad flat, no-lead (QFN) package and the dual flat, no-lead (DFN) package. The DFN package is essentially the same as a QFN package except the DFN package includes leads on two sides of the package instead of four. The no-lead construction provides an IC package having an encapsulation body with leads for electrical connection that do not protrude or extend from the body. That is, the package has leads that are flush with the underside of the encapsulation body and do not extend outwardly, thereby permitting the package to have a compact size. The QFN and DFN packages can be surface-mounted on a printed circuit board (PCB) and can be electrically connected to the PCB at the non-protruding leads on the underside of the encapsulation body.
Leadframe production techniques make use of a leadframe strip having hundreds of die pads, onto which semiconductor chips (also referred to as dies or devices) are mounted and then encapsulated. Packages such as the QFN and the DFN are sometimes referred to within the semiconductor industry as micro lead frame packages (MLF or MLP). The semiconductor devices are typically mounted onto die pads of the leadframe strip using epoxy or other adhesives. The leadframe strip includes finger-like connecting tabs (also called leads or contact pads) that extend from the frame strip around each of the die pads. A single leadframe strip can include hundreds of die pads and corresponding numbers of leads for each die pad. Each device is then electrically connected to the leadframe by wire bonds that extend from the leads of the leadframe to bonding pads of each device. The leadframe with bonded devices and wire bond connections is then laid flat and encapsulated in plastic or resin, protecting the device and wire connections from environmental effects and leaving the exposed leads on the underside of the leadframe. The encapsulated devices are then segmented from the leadframe, either by saw cutting or stamping, thereby producing the MLPs in large quantity. The leadframe production technique can provide encapsulated chips at lower cost and with smaller physical sizes as compared with non-leadframe production techniques. The leadframe production techniques, however, cannot generally provide exposed contacts on packages in an array configuration, thus limiting the number of exposed leads that can be provided.
FIG. 1 is a cross-sectional view of a simplified MLP 100. The MLP 100 includes an IC die 102 attached to a die pad portion 104 of a leadframe 106 by an adhesive 108. Bonding pads 110 are formed on the top of the IC die 102 and are connected to leads 112 of the leadframe 106 by wirebonds 114. An encapsulant material 120 covers the package including the IC die 102, the wirebonds 114, and the upper surfaces of the die pad 104 and leads 112. The die pad 104 and leads 112 are exposed on the bottom of the package 100, thereby facilitating heat dissipation from the IC die 102 external to the package and decreasing the overall thickness of the package. A half-etched portion 122 can be formed on the sidewall surface of the die pad 104 and leads 112 to better mechanically secure the die pad and leads to the encapsulant material, which flows underneath the half-etched portion 122.
Typically, many MLP 100 IC packages are formed in a matrix pattern on a leadframe strip, and the MLP is typically encapsulated using one of two different techniques, which can be referred to as block molding or matrix molding. Packages encapsulated using a block molding technique are typically separated from the lead frame strip using a sawing process while packages encapsulated using a matrix molding technique are typically separated from the lead frame using a punch process. To accommodate the different types of encapsulation and singulation processes, leadframe strips are specifically designed for one or the other technique. Thus, “leadframe” refers to production techniques associated with IC packages mounted on die pad attachment strips, and “no-lead” or “leadless” packages refers to a produced IC package in which the encapsulation body has connecting leads that are flush with the body and thus appear to be without leads.
FIG. 2 is a view of the underside of the FIG. 1 MLP 100. The leadframe portion 106 is visible in FIG. 2, and shows that the MLP includes a row of contacts 202 on each edge of the MLP encapsulation body that comprise the ends of the leads 112 first shown in FIG. 1. Thus, the MLP illustrated is a QFN package. Although each edge of the MLP 100 is shown with three contact leads per edge, it should be understood that a greater number of contact leads are typically included on each edge. For example, MLP configurations typically include configurations with sixteen leads in total, with four leads per body edge for a QFN.
Current chip design trends are to provide an increasing number of exposed leads per chip package. At the same time, trends are for decreasing the overall size of the encapsulation package. Thus, a greater number of exposed leads in a smaller package has resulted in much greater density of exposed leads. Increasing the number of rows of exposed leads per edge would help accommodate greater lead density of the packages. For leadframe production techniques, it is possible to provide two rows of contacts per package edge. That is, each edge includes an outer row of contacts along the outside edge of the chip package and also includes an inner row of contacts adjacent the outer row, typically offset from the outer row in a staggered configuration. For example, an encapsulated chip package with two rows of contacts along an edge is described in U.S. Pat. No. 6,229,200 and another encapsulated chip package with two rows per package edge is described in U.S. Pat. No. 6,838,751.
MLPs with two rows of exposed leads per package edge can be produced by constructing the leadframe with staggered leads connected to the leadframe by half-etch portions, such that the leads include an outer row of contacts and an adjacent, parallel but staggered row of inner contacts. The outer row of contacts are located around the periphery of a leadframe member, extending from the leadframe strip, and the inner row of contacts are located around the periphery of the die pad of the leadframe member. An IC device is placed in the center of each leadframe die pad structure and is then wire-bonded to the leads (contact tabs) of the leadframe in alternating fashion, one contact having a wire bond to an outer contact pad, the next contact having a wire bond to an adjacent inner contact pad, and so forth. Segmentation of the bonded chips from the frame, isolating the contact pads from the leadframe, is followed by encapsulation and other post-encapsulant processing to produce the finished two-row packages.
The two-row leadframe construction increases the number of available contacts for each chip package. The two rows of connecting tabs can be provided in the leadframe by half-etched fingers that extend from the frame to the inner row from one direction and extend from the frame to the outer row from another direction. For the leadframe construction, at the start, the connecting tabs must be physically connected to each other and the frame. During production, the half-etched fingers are removed, thereby physically and electrically isolating the connecting tabs from each other, and providing two rows of exposed leads. Unfortunately, two rows of exposed leads are the practical limit of current leadframe production technology, given the need for physically connecting the contact tabs to the leadframe during the production process. Nevertheless, the trend for increasing the number of lead contacts on a chip package creates a need techniques that accommodate two or more numbers of rows of exposed leads per chip package, in combination with efficient and economical leadframe production techniques. The present invention satisfies this need.